Modes of Nutrition

Modes of Nutrition:

Two modes of nutrition are recognized in organisms. These are autotrophic or photosynthetic and holotrophic or ingestive.

(1) Autotrophic Nutrition- Green plants synthesise their organic food from CO₂ and water in the presence of sunlight. The process is called photosynthesis and the mode of feeding is termed autotrophic nutrition. Being a characteristic of plants, it is also called holophytic nutrition.

Certain bacteria prepare their organic food from CO₂ and hydrogen compounds other than water, such as hydrogen sulphide and methane. They may get energy for the synthetic process from sunlight (photosynthesis) or from the oxidation of inorganic materials (chemosynthesis). Such bacteria are called photosynthetic or phototrophic or phototrophs and chemosynthetic or chemotrophic or chemotrophs respectively. The organisms capable of autotrophic nutrition are called autotrophs.

(2) Heterotrophic Nutrition- The modes of nutrition other than autotrophic are called heterotrophic and the organisms having heterotrophic nutrition are called heterotrophs. Heterotrophic nutrition is of the following two types-

• Saprotrophic or Saprozoic- In this case, heterotrophs absorb fluid food through the body surface. It is called saprozoic nutrition in animals. For Example- tapeworm, and saprophytic nutrition in plants example- fungi. Lichens, made of an alga and a fungal member, are the best examples of symbiotic relationships. The alga provides nutrition to the fungus, which in turn absorbs water and minerals and makes them available to the fungus. In this way, both the partners derive benefits from each other. Bacteria living in the root nodules of legumes plants and nitrogen-fixing blue-green algae are other examples of symbiosis.
• Holotrophic or Holozoic- It is a characteristic of animals where the animals take solid food through the mouth. The animals having this type of nutrition are called holotrophs. The animals who depend for their food on plants are called herbivores, those who prey upon other animals are called carnivores and animals which depend on plants as well as on other animals are called omnivores. Certain organisms, like Euglena, have more than one mode of feeding and are called mixotrophic and this mode of feeding is termed mixotrophic nutrition.